Melanie Daniels rents a small motor boat to sneak the love birds over to where Mitch Brenner's house is. There is a dock that she was easily able to access and tie off her boat and is easily able to get back across the bay too. Melanie is invited to Cathy's birthday party the next day and does attend. During the party, Mitch and Melanie are walking up a hill behind the house which is clearly visible in the shot. Take a look at the dock she tied her boat off to, the bay is nearly all gone. In a 24 hour span the bay somehow emptied, maybe it's a low tide.